This is not about mass surveillance of mail, this is about the sorts of targeted surveillance the US Postal Inspection Service uses to catch mail thieves:

To track down an alleged mail thief, a US postal inspector used license plate reader technology, GPS data collected by a rental car company, and, most damning of all, hid a camera inside one of the targeted blue post boxes which captured the suspect’s full face as they allegedly helped themselves to swathes of peoples’ mail.